In recent years, restrictions on chemical substances harmful to the environment have become increasingly stringent, and the restrictions on solder materials used to join electronic parts or the like to a substrate are no exceptions. Pb (lead) has been used as a major component of solder materials since old times, but has already been designated as a restricted substance by, for example, RoHS Directive. Therefore, solders containing no Pb (hereinafter, also referred to as “Pb-free solders”) have been actively developed.
Solders used to join electronic parts to a substrate are broadly divided into high-temperature solders (about 260 to 400° C.) and low- and medium-temperature solders (about 140 to 230° C.) based on their service temperature limits. As for low- and medium-temperature solders, Pb-free solders mainly containing Sn have already been practically used. For example, Patent Document 1 discloses a Pb-free solder alloy composition containing Sn as a major component, 1.0 to 4.0% by mass of Ag, 2.0% by mass or less of Cu, 0.5% by mass or less of Ni, and 0.2% by mass or less of P. Further, Patent Document 2 discloses a Pb-free solder alloy composition containing 0.5 to 3.5% by mass of Ag, 0.5 to 2.0% by mass of Cu, and the balance Sn.
On the other hand, high-temperature Pb-free solder materials also have been developed in various organizations. For example, Patent Document 3 discloses a Bi/Ag brazing filler material containing 30 to 80% by mass of Bi and having a melting temperature of 350 to 500° C. Further, Patent Document 4 discloses a solder alloy obtained by adding a binary eutectic alloy to an eutectic alloy containing Bi and by further adding an additive element thereto, and describes that this solder alloy is a quaternary or higher solder, that is, a multi-component solder, but it is possible to adjust its liquidus-line temperature and to reduce variations in composition.
Patent Document 5 discloses a solder alloy obtained by adding Cu—Al—Mn, Cu, or Ni to Bi, and describes that when such a solder alloy is used to join a power semiconductor device having a Cu surface layer to an insulator substrate having a Cu surface layer, an undesired reaction product is less likely to be formed at a joint interface between the solder and each of the Cu layers so that the occurrence of defects such as cracks can be suppressed.
Patent Document 6 discloses a solder composition containing, based on the total mass of the solder composition, 94.5% by mass or more of Bi as a first metal element, 2.5% by mass of Ag as a second metal element, and a total of 0.1 to 3.0% by mass of at least one selected from the group consisting of 0.1 to 0.5% by mass of Sn, 0.1 to 0.3% by mass of Cu, 0.1 to 0.5% by mass of In, 0.1 to 3.0% by mass of Sb, and 0.1 to 3.0% by mass of Zn as a third metal element.
Patent Document 7 discloses a Pb-free solder composition containing a Bi-based alloy containing at least one of Ag, Cu, Zn, and Sb as an accessory component and 0.3 to 0.5% by mass of Ni, and describes that this Pb-free solder has a solidus-line temperature of 250° C. or higher and a liquidus-line temperature of 300° C. or less. Further, Patent Document 8 discloses a binary alloy containing Bi, and describes that this binary alloy has the effect of suppressing the occurrence of cracks in the inside of a soldering structure.
Patent Document 9 discloses a Bi alloy having a melting temperature of 270° C. or higher and containing 0.2 to 0.8% by mass of Cu and 0.2 to 0.02% by mass of Ge. Patent Document 10 discloses a Bi alloy having a solidus-line temperature of at least 260° C. and containing 2 to 18% by mass of Ag. Patent Document 11 discloses a Bi alloy having a solidus-line temperature of 262.5° C. or higher and containing 82 to 98% by mass of Bi.